The natural resources used by society are limited by various factors. Why is coal considered a nonrenewable resource?
a. Once coal is used up, it can never be replaced.
b. Coal is a nonrenewable resource, because it comes from the impact of meteors with the Earth.
c. Coal was a renewable resource, until it was mismanaged by humans.
d. Coal is considered a nonrenewable resource, because it takes millions of years to form.